The present invention relates to a thermostatic expansion valve which is extensively but primarily used for a refrigeration cycle system such as automotive air conditioning apparatus.
Such a thermostatic expansion valve is included in a refrigeration cycle and is for expansion of a refrigerant which is contained in the refrigeration cycle. The thermostatic expansion valve in an earlier technology comprises a refrigerant passage for guiding the refrigerant in a predetermined direction, a valve seat dividing the refrigerant passage into a high-pressure chamber and a low-pressure chamber, a valve body movable in the high-pressure chamber for adjusting a flow of the refrigerant in cooperation with the valve seat, and a control arrangement for controlling movement of the valve body in response to temperature of the refrigerant.
With reference to FIG. 4, description will be made as a thermostatic expansion valve of the type described above. The thermostatic expansion valve is generally used for automotive or car air conditioning system employing a volume valuable compressor of a piston stroke controlling type such as a swash plate type compressor.
The thermostatic expansion valve has a casing 1, an expansion valve unit 2 and a closure member 3 in the casing 1. In a casing 1, there are provided a high-pressure passage 11 which serves as the refrigerant passage directing to an evaporator 4 for a high pressure refrigerant which is discharged from a compressor discharging chamber, low-pressure passages 12, 12 which serve as a passage directing to a compressor suction chamber for a low pressure refrigerant which is discharged from the evaporator 4, and a valve unit insertion portion 13 which is disposed between the low-pressure passages 12. The closure member 3 is located at an upper portion of the valve unit insertion portion 13 such that an end of the expansion valve 2 is adaptable by the use of engagement member.
The expansion valve unit 2 has a valve seat 200a which is located to form a high-pressure chamber 200a and a port 200b in the high-pressure passage 11 of the casing 1, a valve casing 200 disposed at a center of the casing 1 to close a passage between the high-pressure passage 11 and the valve unit insertion portion 13, a valve body 201 which is disposed in the high-pressure chamber 10 and contacted with, and spaced from, the valve seat 200a to open/close a passage directing to the evaporator 4 through the high-pressure passage 11, the valve seat 200a, and the port 200b, a spring 203 for biasing the valve body 201 toward a valve-closing direction (an upward direction in the illustration of FIG. 4) through a guide member 202, and an adjustment screw 204 for adjusting a pressing force of the spring 203. Further, there is disposed a temperature sensing portion 205 which is disposed in the valve unit insertion portion 13 of the casing 1 such that an end portion of the temperature sensing portion 205 is mounted to the closure member 3 and which is disposed in the midst of the low-pressure passage 12 directing from the outlet portion of the evaporator 4 to the suction (or inlet) chamber of the compressor and, in addition, a diaphragm 206 which is displaced in accordance with pressure difference between the inner pressure of the temperature sensing portion 205 and the pressure of the outlet of the evaporator 4, a transmission rod 207 which is displaceably supported to the valve casing 200 such that one end thereof is contacted with the diaphragm 206 and the other end is provided with the valve body 201 so that the valve body 201 is opened/closed in accordance with the displacement of the diaphragm 206, and a spring 208 for urging the transmission rod 207 toward the diaphragm 206. A combination of the temperature sensing portion 205, the diaphragm 206, the transmission rod 207, and the spring 208 is referred to as the control arrangement.
The expansion valve unit 2 has a passage 200c at the valve casing 200 so that the diaphragm 206 receives, or effected by, the pressure from the evaporator 4 by the passage 200c.
Within the temperature sensing portion 205 which is exposed to the refrigerant from the outlet of the evaporator 4, a refrigerant (R134a) and an adsorbent (oil) is sealed therein, and the pressure in the temperature sensing portion 205 is set to be varied in accordance with the temperature of the refrigerant from the outlet of the evaporator 4.
By the structure described above, a superheat degree characteristic is determined by a force due to a difference of the pressure added to both surfaces of the diaphragm 206 (that is, difference between a force for pressing the diaphragm 206 toward the valve body 201 and a force acting in the valve opening/closing direction of the valve body 201), and a spring force of the spring 203.
FIG. 5 shows a characteristic of temperature (.degree. C.)-pressure (kg/cm.sup.2 G) under a predetermined pressure condition of the inlet of the thermostatic expansion valve described above. In FIG. 5, the characteristic C1 with respect to the expansion valve represents a linear line which shows that a pressure proportionally increases as the elevation of the temperature, whereas the characteristic C2 with respect to the refrigerant (R134a) represents a curve which shows that a pressure gradually varies and increases as the elevation of the temperature. As seen from FIG. 5, it is prescribed that the characteristic C1 extends across the characteristic C2.
Namely, in comparison between characteristic C1 and characteristic C2, if temperatures are compared with reference to pressure elevation up to 2.0 kg/cm.sup.2 G, the temperature of characteristic C1 represents .degree. C. whereas the temperature of characteristic C2 represents a temperature value slightly higher than .degree. C. However, if temperatures are then compared with reference to pressure elevation up to 2.7 kg/cm.sup.2 G, the temperature of characteristic C1 represents 10.degree. C. whereas the temperature of characteristic C2 represents a temperature value lower than 10.degree. C. by .DELTA.T. Thus, a relationship of the temperatures relative to the pressure is reversed at a temperature above .degree. C. and around 1.2.degree. C. to form a break-even or cross-over point. This is aimed to obtain restriction of hunting of an expansion valve especially at a low and middle temperature range and returning of the refrigerant (including an oil) to the compressor, because the compressor is in a continuous operation to a low outdoor temperature range and a circulation amount of the refrigerant is extremely reduced in this region.
In case of the thermostatic expansion valve described above, the characteristic C1 of the expansion valve is located at a higher position than the characteristic C2 of the refrigerant in the region of lower temperature than the cross-point. In this state, the expansion valve is always opened, and the high pressure side and the low pressure side are not closed or cut off even in the suspended state of the compressor and, accordingly, the refrigerant which has been trapped at the high pressure side due to the change of the temperature in and out of the vehicle is moved to the low pressure side through the expansion valve so that it is likely that a great amount of the refrigerant is stored in the interior of the compressor itself and in its suction passage. If, in this state, the compressor is driven, liquid compression is generated to cause serious problems such as damage and breakage in the compressor. Accordingly, it is necessary that the cases that the liquid refrigerant is delivered from the thermostatic expansion valve side to the compressor itself and/or its suction passage must be avoided.